1. Field of the Invention
The present invention relates to a high-voltage resistance wire for use in an engine ignition device, for instance.
2. Description of the Prior Art
The background of the present invention will be explained with respect to its application to an engine ignition device installed in an automotive vehicle.
In an engine ignition device, a high-voltage (e.g. 25 to 30 kv) resistance wire is used to obtain an appropriate resistance. Examples of the prior-art high-voltage resistance wire of this sort are disclosed in Japanese Published Unexamined Pat. Appl. No. 54-140190, for instance, as shown in FIGS. 1 and 2 of the present specification.
In FIG. 1, the high-voltage resistance wire is made up of a resistive conductor 1, an insulating layer 2 covering the resistive conductor 1, and a protective sheath layer 3 for protecting the insulating layer 2. In this prior-art resistance wire, however, since the protective sheath layer 3 is formed in close contact with the insulating layer 2, it is necessary to decrease the outer diameter of the resistive conductor 1 or to increase the outer diameters of the insulating layer 2 and the protective sheath layer 3, in order to decrease the electrostatic capacitance between the resistive conductor 1 and the protective sheath layer 3. Therefore, there exists a problem in that it is difficult to lower the electrostatic capacitance of the resistance wire, and therefore ignitability of an engine ignition device using the resistance wire is low.
In addition, since the protective sheath layer 3 of this prior-art resistance wire is made of rubber, the mechanical strength (e.g. tear resistance) of the protective sheath wire 3 is subject to degradation, in particular when temperature rises within an engine housing. There thus exists another problem in that the protective sheath layer 3 is easily torn away from a metallic end terminal T, as shown in FIG. 3, attached to an wire end for facilitating connection of the wire to another element; that is, the tearing strength or the tear resistance of the protective sheath layer is not sufficiently high against the wire end terminal.
FIG. 2 shows another example of the prior-art high-voltage resistance wire. This wire is made up of a resistive conductor 1, a protective sheath layer 3, and many insulating spacers S intervening between the conductor 1 and the sheath layer 3 to form some spaces 4 therebetween. In this resistance wire, although the dielectric strength is high and the electrostatic capacitance is low, since some spaces are formed by intervening spacers between the conductor 1 and the sheath layer 3, another problem exists in that the wire is bulky without contributing to the space-saving requirements of the engine housing. Additionally, the bendability or flexibility of the wire is low because of the presence of the spacers S.